Chemicals in tobacco may help trigger serious mental illnesses such as schizophrenia, a group of UK researchers find.

Smoking tobacco, already known to cause cancer and stroke, may be also be a contributor to mental illness, a new study suggests.

Researchers at King's College London have found that people who suffer from psychosis are about three times more likely to be smokers.

Although the association between smoking cigarettes and psychosis - particularly schizophrenia - has been acknowledged before, little attention has been directed towards the possibility that cigarettes themselves may increase the risk of psychosis.

The study also showed that daily smokers became psychotic around a year earlier than non-smokers.

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It has long been hypothesised that higher smoking rates among psychosis sufferers could be explained by people seeking relief from boredom or distress, or self-medicating against the symptoms or side-effects of anti-psychotic medication.

But if this were so, researchers would expect smoking rates to increase only after people had developed psychosis.

The King's College London team said in a statement: "These findings call into question the self-medication hypothesis by suggesting that smoking may have a causal role in psychosis."

They stressed they had not conclusively proven that smoking causes psychosis, saying further research must be done.

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But the results did suggest that smoking "should be taken seriously as a possible risk factor for developing psychosis and not dismissed simply as a consequence of the illness," they wrote.

The researchers theorised that changes in the brain's dopamine system may explain the association.

Dopamine is a chemical messenger that helps control the brain's reward and pleasure centres.

"Excess dopamine is the best biological explanation we have for psychotic illnesses such as schizophrenia," said King's College London psychiatric professor Robin Murray.

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"It is possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop."

Dr Michael Bloomfield, clinical lecturer in psychiatry at University College London, said: "Much more research is needed before scientists can say for certain that smoking definitely increases the risk of schizophrenia since it remains possible that people who would go on to develop schizophrenia are more likely to start smoking.

"Regardless of these findings, there is overwhelming evidence that nicotine use through tobacco smoking is one of the most dangerous drug problems in the world. Anyone who needs help in stopping smoking should speak with their doctor."